The percentage of users who install firefox is low because of the inertia of the default. Having Google as the default search engine in firefox certainly didn't help.
Imagine downloading firefox to replace IE or Edge on a fresh Windows install and then immediately witness Chrome ads in your search results.
Mozilla should had disrupted the third party tracking/ads business, when it had the chance, by providing a default ad blocker and severing ties with no-privacy-respecting search engines (before Google disrupted the browsers market that is).
Google's Android browser is doing well by not supporting extensions, why would they miss the chance of additional revenue by not crippling their desktop browser the same way?